To determine a porosity of a subterranean formation, it may be desirable to make several simultaneous measurements. One tool for measuring porosity is based on neutron transport through the subterranean formation. The neutron flux attenuated with distance from the source may depend strongly on the hydrogen content of the subterranean formation. For a neutron source, radioisotopic sources or accelerator based sources are used in existing tools.
If pore spaces are filled by liquid, the higher porosity corresponds to a higher hydrogen index. The detected neutron counts are generally lower in this case. A properly calibrated tool may increase the accuracy of the porosity measurement in liquid-filled formations if the matrix composition is known. However, the measurement may be affected by various environmental conditions.
On the other hand, the same measurement may be less accurate for gas-filled subterranean formations when the hydrogen content in the pore spaces is lower due to the relatively low density of the gas. A density measurement may address this ambiguity. For the same porosity of the subterranean formation, the gas-filled and liquid-filled matrix have different densities.
One environmental condition that may affect the porosity measurement is tool position or stand-off in the borehole. The stand-off and the borehole fluid (liquid or gas) may impact the count rate in radiation detectors.